The Koh Samui has postcard sailing conditions from May through August, while the rest of Asia enjoys the wet season delivered by the southwest monsoon. The mountainous terrain of the Malay peninsular to the west exhausts the monsoon winds of their rain and leaves Koh Samui and its surrounding islands basking in the sunshine with flat seas and a cool, steady breeze.
May through September
Weather conditions remain nicely tropical, with temperatures ranging from 30C in the daytime to 24C during the night. Wind can range from 10 to 25 knots, and although conditions remain mainly dry, afternoon squalls are common and predictable. Sailing conditions are relatively uniform throughout the season and can be more challenging than Koh Chang and Phuket. The islands are 20nm apart, so the passage between islands can take most of the day. But the islands are big, so once you arrive, it's easy to spend several days at just one, so there is no need to be sailing all day long every day; there's plenty of bay hopping to do.
Overall, the Samui cruising ground is a great place to charter a yacht, with moderately challenging sailing conditions by day and beautiful tranquil anchorages by night. It is, without doubt, the best place to go sailing in Asia from May through September.